Polymeric materials



Patented Dec. 18, 1951 UNITED STATES PATENT OFFICE POLYMERIC MATERIALSAllen E. Poison, Tallmadge, Ohio, assignor to E. E. du Pont de Nemours &Company, Wilmington, Del., a corporation of Delaware No Drawing.Application December 31, 1948, Serial No. 68,761

Polyacrylonitrile possesses a number of prop erties which makes itvaluable for many applications, such as, for example, for the formationf of fibers and films and other shaped articles. More specifically,fibers of polyacrylonitrile have good tensile strength but in otherproperties they are not entirely satisfactory. One deficiency of thesefibers is their low receptivity to certain types of dyes. Because ofthis poor dye receptivity polyacrylonitrile fibers cannot be used forthe preparation of fabrics having certain desired colors. This is aserious defect from a commercial standpoint.

This invention has as an object a new and useful composition of mattercomprising a modified polyacrylonitrile. A further object is theproduction of solutions which upon extrusion into a coagulating bathyield improved shaped articles comprising a modified polyacrylonitrile.A further object is a shaped article, such as a film, filament or thelike which comprises a modified polyacrylonitrile and which hasexcellent dye receptivity. Further objects reside in methods forobtaining these shaped articles.

I have found that the above objects can be accomplished and that theabove-mentioned disadvantages of the prior practice can be overcome bymeans of a partially hydrolyzed polyacrylonitrile. The partiallyhydrolyzed poly-,- acrylonitriles of this invention can be obtained ofthe nitrile groups may be hydrolyzed further to carboxyl groups, butunder preferred conditions the hydrolysis to carboxyl groups isinsufilcient to impart to the product solubility in dilute alkali.

In a preferred manner of carrying out this invention, finely dividedpolyacrylonitrile is added to sulfuric, nitric or phosphoric acid of aconcentration sufiicient to dissolve the polymer- The particularconcentration of acid employed depends on the particular acid and par-"ticular temperature being used. Sulfuric acid by dissolvingpolyacrylonitrile in one of .thewellj' known -ic acids of nitrogen,phosphorus and sulfur, namely, nitric acid, phosphoric acid and sulfuricacid. The products obtained in' accordance with the practice of thisinvention consist of modified, coagulated polyacryloni The mostdesirable products are those trile, obtained by continuing thehydrolysis until at of 75% to 85% strength, i. e., 75% to 85% H2804 and25% to 15% water, is preferred for dissolving polyacrylonitrile attemperatures between 10' and 25 C. Nitric acid of to strength issuitable for dissolving polyacrylonitrile at temperatures between 0 and35 C. Phosphoric acid of approximately100% concentration dissolvespolyacrylonitrile at temperatures of 0 to 35 C. The mixture ofpolyacrylonitrile and the inorganic oxy acid is stirred until a uniformsolution is obtained. The nitrile groups in the polyacrylonitrile insolution are gradually hydrolyzed and converted to amide groups by theoxy acid, the rate of hydrolysis or} conversion depending primarily onsuch factors as the kind and concentration of the oxy acid and thetemperature of hydrolysis. The resulting solution is held within theabove-mentioned temperature ranges until the desired degree ofconversion is obtained. When sulfuric acid of to, 30% strength is used,2% to 75% conversion of nitrile groups to amide groups is obtainedinfitohours at 0 to 5 C. When .70 %,-nitric acidis employed, similarconversions areobtain'fedinfi to 100 hours'at '25 0. With.100,%1".lpliosphoric acid at 25 0. the rate of conversion approximatelythe same as with least 2% and not more than 75% of the nitrile groupsoriginally present have been hydrolyzed,

nitric acidat-the same temperature. I from vthe standpoint'ofc'oncentrationof solids at spinning viscosity, solutions .ofpolyacrylonitrile in nitric acid are especially suitable because nitric,acid and thereby converted primarily into amide groups.

' More specifically, the practice of this invention involves dissolvingpolyacrylonitrile in an acid of the above-mentioned kind and extrudingthe solution into a liquid coagulating bath which is a non-solvent forthe dissolved polymer but a solvent for the acid, some water beingpresent either in the acid solution of polyacrylonitrile or in thecoagulating bath. Although the nitrile groups originally present areconverted primarily into amide groups, a small number is-capab le ofdissolving considerably'higher concentrationsof polyacrylonitrile thansulfuric or phosphoric acids. Phosphoric acid is the least desirablefofthe three from this standpoint. The partially hydrolyzedpolyacrylonitrile is isolated by extruding the oxy acid solution in thedesired shape,e. g., in the form of films or fibers, into an aqueouscoagulating bath at.

0'-60 C. Informing fibers,- those polymers in which from 25% to 65% ofthe nitrile groups are converted to amide groups are especiallypreferred since the resulting fibers have good receptivity to direct,vat, acetate, and acid type dyes. The coagulated films and fibers arethen washed free of the salts and acid present, and dried. The resultingfilms or fibers can be sub- Jected to conventional aftertrea'tments suchas drawing, or stretching, to develop optimum physical properties.

The polyacrylonitrile used in the practice of this invention can beprepared by any of the conventional methods for polymerizingacrylonitrile. Preferably the polymer is prepared by the to furtherdrawing treatment in hot oil or hot air prior to evaluation. In one casethe sized yarn is passed through mineral oil at 205 C. and drawn to 9times its original length. Another sample of the sized yarn is passedthrough air at 215 C. and drawn to 12 times its original length. In athird sample the yarn is drawn to 12 times its original length-in air at215 C., after which it is relaxed 17.5%. The properties of 10 theseyarns are summarized in the following table:

Hot-drawn yarns of Example I Yarn Properties Tenacity/E1011 tion g. p.d./ Work Re- Method of Drawing l Shrinkage D e Race in Boiling ,33, ui mm Water, Elam: a acid and Dry Loop wet Per Cent (Per cent) direct dyesl3 2. 0/2 2. 6/23 52 8i Good.

Hot All 8.1/4. 6 4.1/23 82 Hot Air and Relaxed 3. 4/44 2. 8136 8. 214530 87 ammonium persulfate catalyzed polymerization 25 of acrylonitriledissolved or emulsified in water. It can also be prepared by otherpolymerization reactions such as, for example, by the emulsion typepolymerization reaction described in U. S. 2,160,054 to Bauer et al.

The invention is illustrated in further detail by the following examplesin which the proportions of ingredients are expressed in parts by weightunless otherwise specified.

EXAMPLE I One hundred and ten parts of powdered polyacrylonitrile havinga relative viscosity of 1.338 measured in 0.1% concentration (0.1 g. per100 ml.) in dimethylformamide at 25 C. is stirred into 2,640 parts of75% sulfuric acid maintained at -6 C. by means of an ice bath andstirring is continued until a uniform solution is obtained (about 18hours). This solution is then stored at 2-3 C. and maintained under apressure of hydrolysis of the mmle groups in the Dalila? -20 mm. ofmercury to remove air bubbles. After the solution has aged for 48 hoursfrom the time the polyacrylonitrile was first added to the sulfuric acida portion amounting to 830 parts is extruded from a stainless steel bombthrough a spinneret having 60 holes, each 0.003" in diameter, into asaturated aqueous solution of sodium acid sulfate at C. The pH of thecoagulating bath is maintained between 2 and 4 by the periodic additionof sodium hydroxide solution during the spinning of the yarn. Thepolyacrylonitrile coagulates in the form of yarn in the sodium bisulfatebath. The yarn is then passed under tension through a boiling aqueous20% solution of sodium sulfate to reduce gel swelling and to draw it.The yarn is drawn, or stretched, to 1.52 times its original lengthduring passage through this hot sodium sulfate solution. The stretchedyarn is wound on bobbins, washed in running water and dried at 20-30 C.The resulting yarn is colorless and glossy. Analysis of samples of theyarn for carbon, hydrogen, and nitrogen gives the following data: Found,N. 23.43%; C. 61.70%; H, 6.25%. The per cent hydrolysis of the nitrilegroups to amide groups in the polymer calculated from nitrogen analysisis 37.5%.

The yarn from the above example is given a twist of 2 or 3 turns perinch, treated with a'conventional rayon viscose size, and then subjectedAll yarns previously drawn 1.52 times their original length in 20%aqueous sodium sulfate solution at The tenacity and work recoveryproperties of the yams described in this specification are determined bystandard testing procedures for yarn. Shrinkage is determined bymeasuring a segment of the yarn before and after a 3-minute immersioninboiling water. Dye receptivity is determined qualitatively by immersinga sample of the yarn held taut on a wire form in a hot dye bath for onehour, followed by washing the sample in hot soap solution for 30minutes. The

dye receptivity is judged by the amount of color retained by the yarn incomparison with, control samples of unhydrolyzed polyacrylonitrile yarn.

EXAMPLE II Another 830-part portion of the 75% sulfuric acid solution ofpolyacrylonitrile described in Example I is aged for 72 hours at 2-3 C.and spun into yarn by the procedure of that example.

. Analysis of this yarn gives the following results:

N, 21.75%; C, 58.12%; H, 6.60%. The per cent lonitrile calculated fromnitrogen content is 63%. Yarn stretched 1.52% in the hot sodium sulfatebath, twisted and sized as in Example I, and then drawn to 9 times itsoriginal length in mineral oil at 205 C. exhibits the followingproperties:

Dry tenacity 2.3 g./denier at 3% elongation, loop tenacity 2.1g./den.ier at 2% elongation, wet tenacity 2.0 g./denier at 36%elongation, 72% work recovery at elongation, shrinkage of 68% onimmersion in boiling water, and good dye receptivity to both acid anddirect dyes. EXAMPLE III Another 830-part portion of the sulfuric acid50 solution of polyacrylonitrile of Example I is aged at 2-3 C. for atotal of 96 hours and then spun into yarn in the same way as in thepreceding two examples. Analysis of this yarn shows it to contain 21.15%nitrogen, 56.26% carbon, and

6.60% hydrogen. The degree of conversion of nitrile groups to amidegroups calculated from nitrogen content is 73%. Yarn treated and drawnin exactly the same way as that of Example II has the followingproperties: Dry tenacity 3.0 g./denier at 13% elongation. loop tenacity75 direct C1288.

EXAMPLE IV Fifty parts of polyacrylonitrile having a relative viscosityof 1.218 measured in 0.1% concentration (0.10 g. per 100 ml.) indimethylformamide at 25 C. is dissolved in 950 parts of 76% sulfuricacid at 3-6 C. in the manner described in Example I. The resultingsmooth solution is aged for a total of 24 hours at 3-6 C. and then spuninto a saturated aqueous solution of sodium bisulfate at 20 C. by theprocedure described in Example I with the exception that the yarn isdrawn to 3.1 times its original length during passage through the hotsodium sulfate solution. Two washed, twisted, and sized samples of thisyarn are subjected to a further drawing to 4 times and 6.4 times,respectively, the original length of the yarn in oil at 200 C. Thephysical properties of these yarns are summarized in the followingtable. Analysis of the yarn: Found C, 64.01%; H, 6.07%; N, 24.46%; 8.0.045%. Calculated for one nitrile group out of five hydrolyzed to anamide group CH17N5OZ C, 63.6%; H, 6.00%; N, 24.7%.

Hot-drawn yams of Example IV Yarn Properties Work Recovery (From 34%Elongation), Per Cent Hot Oil Draw Tenacity/Elongation, g. p. d./

Ratio 1 Per Cent 1 All yarns previously drawn to 3.1 times theiroriginal length in boiling aqueous sodium sulfate during spinning.

EXAIVIPLE V A 5% solution of polyacrylonitrile in 76% sulfuric acid isprepared by stirring 40 parts of polyacrylonitrile having a relativeviscosity of 1.218 measured in 0.10% solution (0.10 g. per 100 ml.) indimethylformamide at C. into a mixture of 152 parts of water and 608parts of. 95% sulfuric acid maintained at 345 C. by means of an icebath. A clear, amber-colored solution is obtained in about 8 hoursstirring. After storing at 3-5 C. for 12 hours more, the solution isspread on a glass plate which is then immersed in water at'roomtemperature. A good, tough film of partially hydrolyzedpolyacrylonitrile, which can be handled without tearing, is formed.

EXAMPLE VI An aqueous solution of 1400 parts of 84% sulfuric acid iscooled with stirring in an ice-salt bath until the temperature of theresulting thick slurry of crystalline sulfuric acid monohydrate drops to6 C. Simultaneously, a slurry of 96 parts of finely powderedpolyacrylonitrile having a relative viscosity of 1.338 in 0.10% solution(0.10 g. per 100/ml.) in dimethylformamide at 25 C. in 904 parts of cold(-2 C.) 61% aqueous sulfuric acid is prepared. The polyacrylonitrileslurry is added rapidly to the stirred crystalline sulfuric acidmonohydrate slurry, the temperature of the mixture rising quickly to 6C. then falling to 4 C. within 3 minutes. The icesalt bath is replacedby an ice-water bath and the mixture maintained under reduced pressureat 03 C. with continued, slow stirring for two hours. The resultingclear viscous solution, which is practically free from air bubbles, isstored at atmospheric pressure in an ice bath for an additional 66hours, whereupon it is spun into 60-filament yarn in a coagulating bathconsisting of 40% aqueous sulfuric acid at 45 C. by the generalprocedure described in Example I. The resulting yarn, stretched to 1.3times its original length during passage through boiling water, andsubjected to the usual finishing and twisting treatments, can be drawnon a hot pin at 170 C. up to ten times its original length. The yarnstretched to 1.3 times its original length has a shrinkage of 26-42% inhot water. The nitrogen content of this yarn, 24.58 (determined byKjeldahl analysis), indicates the polyacrylonitrile to be 21.5%hydrolyzed.

EXAMPLE VII A solution of polyacrylonitrile innitric acid is prepared bythoroughly agitating a mixture of 6.8 parts of polyacrylonitrile and68.5 parts of concentrated nitric acid (70% HNOa minimum) at roomtemperature (about 25 C.). After the polymer is completely in solutionagitation is continued for one hour and the solution is allowed to standat 25 C. for 15 hours. The solution is then slowly poured in a finestream into hot water which i kept rotating in a container to preventthe freshly formed thread from sticking to itself while it is stillsticky. On removal from the water, the thread of partially hydrolyzedpolyacrylonitrile is observed to be clear, of good color, elastic,tough, and capable of being colddrawn. After drying under tension in theopen air the filament is clear, of good color and fairly tough.

Two phenomena appear to operate when polyacrylonitrile is dissolved innitric, phosphoric or sulfuric acid: (a) direct solution, and (b)partial hydrolysis. These phenomena are inseparable, but under someconditions one will predominate, while under different conditions theother will predominate. The kind and concentration of the acid and theoperating temperature determine which of these phenomena willpredominate at a given stage of the reaction, and, together with thetime of treatment, will control the degree of hydrolysis of the nitrilegroups. When sulfuric acid of to 100% strength, i. e., 75% to 100% H2804and 25% to 0% water, is employed at temperatures of 10 to 50 C., theacid apparently acts first as a solvent for the unmodifiedpolyacrylonitrile, and-then gradually hydrolyzes the nitrile groups inthe dissolved polymer. Nitric acid of 55 to 70% strength used attemperatures of 0 to 25 C. and 100% phosphoric acid used at the sametemperatures acts in a similar manner. On the other hand, when sulfuricacid of 50% to 75% strength is employed at elevated temperatures, e. g.,60 to C., the acid apparently first hydrolyzes the nitrile groups nearthe surface of the polymer and then dissolves the polymer, the partiallyhydrolyzed polymer being soluble in sulfuric acid of lower strength thanrequired for unmodified polyacrylonitrile. Nitric acid of 50% strengthand lower and phosphoric acid of 90% strength and lower act in a similarmanner. Thus, the acids of lower concentrations. e. g., sulfuric acid of50 to 75% concentration, nitric acid of 40 to 50% concentration andphosphoric acid of 75 to concentration, can be used to dissolvepolyacrylonitrile which has previously been partially hydrolyzed. Therather drastic condiitons required to dissolve polyacrylonitrile in theoxy acids of lower concentrations mentioned above tend to causehydrolysis of the nitrile groups beyond amide groups to imide groups andeven to carboxyl groups as evidenced by the fact that polymers obtainedin this manher are soluble in dilute aqueous alkali but are insoluble inwater. When sulfuric acid solutions of polyacrylonitrile are to be heldat low temperatures, e. g'., at to C., it is preferable to use sulfuricacid of 75% to 80% concentration as the solvent since sulfuric acidmonohydrate, H2SO4.H2O, which has a melting point of 8.6" C., may tendto crystallize from acid of more than 80% strength at thesetemperatures. Sulfuric acid containing excess sulfur trioxide willdissolve polyacrylonitrile, but the use of such strong acid isundesirable since it causes some charring of the dissolved polymer.

when water is not present in the solvent acid or is present in onlysmall amounts, it should be contained in the coagulating bath in amountsufficient to make the bath a non-solvent for the dissolved polymer. Forinstance the coagulating bath for sulfuric acid solutions ofpolyacrylonitrile should contain about 60% water or more.

In the practice of this invention on a commercial scale, it ispreferable, for economic reasons, to use as solvent for thepolyacrylonitrile an acid of the lowest concentration possible since inthe formation of films or fibers the polyacrylonitrile/inorganic oxyacid solution is extruded into aqueous baths, and obviously the higherthe concentration of the acid in the solution the greater will be theamount of acid to be recovered from the coagulating bath. Therefore, inorder to reduce to the minimum the amount of acid necessary to recover,the unmodified polyacrylonitrile can be dissolved at high solidsconcentration in, for example, sulf-uric acid of concentrations of 75%or above and then, after hydrolysis has proceeded to a substantialextent, the solution diluted, for example, to a sulfuric acid content ofas low as 60% in some instances, thus reducing the polymer solutionviscosity sufliciently to' allow it to be extruded.

The coagulation of the sulfuric, nitric or phosphoric acid solutions ofpolyacrylonitrile of this invention can be carried out in a variety ofways. As indicated previously, coagulation is preferably carried out byextruding the acid solution into aqueous baths. Such baths include wateralone; aqueous solutions of salts such as, for example, sodium sulfate,sodium acid sulfate, sodium nitrate, sodium dihydrogen phosphate, andammonium sulfate, in concentration up to the saturation point of thesalt in. water at the temperature at which the bath is being used;anddilute sulfuric, nitric and phosphoric acid, e. g., aqueous acid of5%-40% concentration. Coagulating baths in which the pH is maintainedbetween 2 and 4 by the addition of sodium hydroxide during coagulationof the oxy acid solution give good results. The use of alkaline baths isto be avoided because of the tendency of alkaline ma- 8polyacrylonitriie solutions in aqueous oxy acids containing sufficientwater to inhibit undesirable chemical reaction of the acid with theorganic liquid. Preferably these organic non-solvent liq-- uid baths areused to coagulate poiyacrylonitrile solutions in sulfuric acid of 60% to80% strength or lower, in those cases where a high proportion of thenitrile groups have been hydrolyzed to amide groups. 4

The partially hydrolyzed polyacrylonitrile can be isolated or coagulatedin any desired form, such as, for example, fibers, monofllaments,ribbons, films, sheets, etc. by conventional means for forming suchshaped articles. Thus, as illustrated by the examples, fibers or yarnscan be obtained by extruding the solution through spinnerets having thedesired number and size of openings into the coagulating bath, and filmsor ribbons can be obtained by extruding the terials to cause colorformation in the coagulated polymer. While aqueous baths are preferred,for economic reasons, for the coagulation of the partially hydrolyzedpolyacrylonitrile other liquids which are miscible with aqueoussulfuric, nitric or phosphoric acids but which are non-solvents for thedissolved polymer can also be employed for this purpose. Examples ofsuch non-aqueous liquids which are suitable include organic liquids ofthe following types: ethers, e. g., diethyl ether; lower aliphaticalcohols, e. g., methyl, ethyl and butyl alcohols; and ketones, e; g.,acetone. These non-aqueous liquids are employed in coagulating sulfuricacid solution of the hydrolyzed p acrylonitrile onto a flat supportfollowed by immersing the support and film in a suitable coagulatingbath or by directly extruding the solu.-.

tion in sheet or in ribbon form into the coagulating bath. Furthermore,if a granular product is desired the oxy acid solution can be coagulatedin the non-solvent bath with sufficient agitation to form particles ofthe desired size, and to keep the coagulated particles of the polymerfrom coalescing.

The shaped articles of this invention, i. e., films, filaments, etc, ofpartially hydrolyzed polyacrylonitrile may, if desired, containmodifying agents such as, for example, dyes, pigments, plasticizers,fillers, and the like. However, in such cases the modifying agents arepresent only in minor proportions.

The yarn, fibers or films of hydrolyzed polyacrylonitrile obtained bythe coagulation of the oxy acid solutions of this invention can besubjected to conventional aftertreatments to improve the physical orother properties of such shaped articles. For example, the tensilestrength of yarn is improved by drawing or stretching the wet spun yarn.Yarns of the partially hydrolyzed polyacrylonitriles of this in-'vention, can be drawn or stretched to about 9 times their originallength in the coagulating bath or in hot aqueous solutions of salts suchas sodium sulfate. They can also-be drawn up to 12 times their originallength by stretching them while heated to a temperature of about 200-220C. in air or in mineral oil at 200--21 0 C... or on a metallic pinheated to to 210 C. In general, optimum balance of properties isobtained by subjecting the yarn to a hot drawing treatment followed by arelaxing. treatment, e. g., a relaxation of 10-20% of the max;- imumdrawn length.

These partially hydrolyzed polyacrylonitrile yarns have differentdegrees of water sensitivity depending on the degree of conversion ofnitrile groups to amide groups, the greater the degree of conversion thegreater bein the water sensitivity. While yarns of hydrolyzedpolyacrylonitrile of 25-65% conversion are preferred because of theirgood dye receptivity, such yarns may be too sensitive to hot water forsome applications. In such case the yarns can be subjected to anaftertreatment such as, for example,

with formaldehyde to improve their water resisttension, can be immersedin a solution of 2% formaldehyde, 10% sulfuric acid and 30% sodiumsulfate for 24 hours at 25 C. and then heated for 1 hour at 70 C. infixed lengths in the bath. After washing and drying, yarns treated inthis manner generally shrink less than on immersion in boiling water for3 minutes.

The partially hydrolyzed polyacrylonitriles of this invention isolatedfrom the sulfuric, nitric or phosphoric acid solutions can, if desired,be redissolved in other solvents and formed into shaped objects. e. g.,films and fibers, by conventional means. The properties of a film ofpartially hydrolyzed polyacrylonitrile (34% conversion of nitrile groupsto amide groups in sulfuric acid solution) cast from dimethylformamidesolution are summarized below. For purpose of comparison, the propertiesof a film of the original polyacrylonitrile of the same thickness (2-3mils) and cast from the same solvent are also given.

Partially fllydriolgiglinal yre o y- Property Polyacryloniacrylonitriletrile Water absorption (immersion in water 24 hours at 25 C. -.per cent14.05 1. 85 Tear resistance (modified Elmendori test):

25 0., 50% R. H 10. 9 4.85 25 0., wet ..g 339+ 10. 5 Flexibility (numberof flexes in Pfund Flexor):

25 o R. H 3 1 25 0., wet 182 30 Tensile strength, 25 0., 50% R. H]b./sq. in.. 11, 000 8,300 Stiffness (A. B. T. M. method D747-43T):

25 0., 50% R. H lb./sq. in" 465, 000 545, 000 25 0., wet ..lb./sq. in77,800 365,

The sulfuric acid solutions of partially hydrolyzed poLvacrylonitriledescribed herein are particularly useful for the preparation of fibersand films. Furthermore, fibers of partially hydrolyzed polyacrylonitrilein which from 25-65% of the nitrile groups are converted to amide groupsare especially suitable for the preparation of fabric to be dyed becauseof the good dye receptivity of these particular partially hydrolyzedpolyacrylonitrile fibers.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that this invention is not limited to the specificembodiments thereof except as defined in the appended claims.

Iclaim:

1. A composition of matter comprising polyacrylonitrile in which from25% to 65% of the nitrile groups are converted to amide groups.

2. A shaped article in the form of a filament comprisingpoLvacrylonitrile in which from 25% to 65% of the nitrile groups areconverted to amide groups.

3. A process for obtaining shaped articles which comprises the steps ofdissolving at a temperature of -10 C. to 35 C. polyacrylonitrile in anoxy acid from the group consisting of nitric acid, phosphoric acid, andsulphuric acid, maintaining the solution at said temperature until from2% to 75% of the nitrile groups in the polyacrylonitrile have beenconverted into amide groups, extruding the solution thus obtainedinto aliquid coagulating bath which is a nonsolvent for the dissolved polymerbut which is a solvent for said acid, water being present in at leastone of these two steps.

4. A process which comprises dissolving at a temperature of from 0' C.to 35 C., polyacrylonitrile in aqueous nitric acid containing from to70% HNOa, maintaining the solution at said temperature until from 2% to75% of the nitrile groups in the polyacrylonitrile have been convertedinto amide groups, and then extruding the solution thus obtained into aliquid coagulating bath which is a nonsolvent for the dissolved polymerbut which is a solvent for nitric acid.

5. The process set forth in claim 4 in which said coagulating bath is anaqueous liquid coagulating bath.

6. The process set forth in claim 4 in which said solution is maintainedat said temperature until from 25% to of said nitrile groups areconverted into amide groups.

7. The process set forth in, claim 6 in which said coagulating bath isan aqueous liquid coagulating bath.

8. A composition of matter comprising a solution in nitric acid ofpolyacrylonitrile in which from 25% to 65% of the nitrile groups arec0nverted into amide groups.

9. A composition of matter comprising a solution in aqueous nitric acidcontaining from 55% to HNOa, of polyacrylonitrile in which from 25% to65% of the nitrile groups are converted into amide groups.

' ALLEN E. POLSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Rubber Chemistry andlechnology,vol. 17.

00 April 1944, page 358. COPY in Division 50.

3. A PROCESS FOR OBTAINING SHAPED ARTICLES WHICH COMPRISES THE STEPS OFDISSOLVING AT A TEMPERATURE OF -10* C. TO 35* C. POLYACRYLONITRILE IN ANOXY ACID FROM THE GROUP CONSISTING OF NITRIC ACID, PHOSPHORIC ACID, ANDSULPHURIC ACID, MAINTAINING THE SOLUTION AT SAID TEMPERATURE UNTIL FROM2% TO 75% OF THE NITRILE GROUPS IN THE POLYACRYLONITRILE HAVE BEENCONVERTED INTO AMIDE GROUPS, EDTRUDING THE SOLUTION THUS OBTAINED INTO ALIQUID COAGULATING BATH WHICH IS A NONOSOLVENT FOR THE DISSOLVED POLYMERBUT WHICH IS A SOLVENT FOR SAID ACID, WATER BEING PRESENT IN AT LEASTONE OF THESE TWO STEPS.